I think a really useful feature you could build into your 3D Viewer would be the ability to show different “Model States”, such as “Levels of Detail” (LODs) and things like that with a little drop-down list to select the different model state from the default, also showing info like poly counts for that LOD etc. This could then also be used to show alternative versions of the same model too.
Right now there is no way for your to show your LODs, other than to put them next to the main model and people having to navigate them and them drawing attention away from the main model.
For example, here is my ITB Exocet Trailer and in Sketchfab, I have no way of showing the LODs, the only thing I can do right now is a Render of them all lined up but again, I can’t show that in Sketchfab other than by a link to the render in the description at most, and viewing it in a render all lined up isn’t easy to view them either and having the ability for people to switch between the different LODs in Sketchfab would make things much better at showing them and also show that a model is fully game ready.
Anyways one useful feature I think would help and could be used in other ways as well